1. Field of the Invention
The present invention relates to conductive paste useful to make a conductive material for a ceramic substrate, and particularly as a conductive material for through hole conduction, and a conductor and a ceramic substrate comprising the conductive paste.
2. Description of the Related Art
In a conventional system for forming through hole conduction in a ceramic substrate, as shown in a sectional view of FIG. 1A, a conductive paste 1 comprising Ag/Pd, Ag/Pt or the like is coated by a screen printing method in a through hole 2 having a diameter of 0.3 to 0.5 mm and formed by a mold or laser, and then burned to form a conductor thick film 3 on the side wall of the through hole 2.
Miniaturization of electronic parts and an increase in the density thereof have recently been demanded, resulting in systema in which the diameter of the through hole is decreased to about 0.1 mm, and the through hole is filled with conductive paste, as shown in a sectional view of FIG. 1B.
Electrodes formed on the surface of the ceramic substrate are mainly Cu electrodes having a low resistance value, and excellent reliability such as the resistance to migration. The Cu electrode is formed by burning in a reducing atmosphere.
When the Cu electrode is used as an electrode on the surface of the ceramic substrate, and when Ag/Pd or Ag/Pt is used as a conductor in the through hole, there is a problem in that the conductor in the through hole and the electrode formed on the surface of the ceramic substrate cannot be burned simultaneously, thereby decreasing the yield per unit of time of usable product. Therefore, the demand for using a Cu electrode as the conductor in the through hole has increased.
However, when the through hole having a diameter of about 0.1 mm is filled with Cu paste and burned, there is a problem in that the conductor in the through hole is cut (see FIG. 2A) due to shrinkage or the conductor peels off (see FIG. 2B) the side wall of the through hole, during burning of the paste, thereby lessening the yield and reliability of through hole conduction.